The Buck and Buck-Boost DC-DC converters use a high side (HS) power switch toggling between the rail-to-rail of the main power supply, that is to say between the potential at the ground terminal, namely 0 volts and the potential at the positive supply terminal, namely Vsup. When this HS power switch is a MOSFET, its gate may need to be driven with a voltage above the Vsup level. In the automotive domain, the minimum Vsup level has kept becoming lower and lower over the years, from 4.5 volts to 3.5 volts, and even to 2.5 volts nowadays. In these conditions, it is more and more difficult to generate a sufficient Gate-Source voltage (Vgs) overdrive for the HS MOSFET.
When this Vgs overdrive is not sufficient to conveniently drive the HS MOSFET, the conduction losses across the HS MOS FET (Metal Oxyde Semiconductor Field Effect Transistor) increase and negatively impact the power efficiency. This degradation on the power efficiency as a function of the reduction in Vsup is particularly true for the Buck topology at low values of Vsup because, in this mode, the current sunk from the power supply and the duty cycle become higher (for providing constant output load current).
These drawbacks need to be compensated, e.g., by an over-sizing of the HS MOSFET, and/or by an expensive Charge Pump structure which requires additional pins, and/or additional external capacitors.
The document entitled “A 4-Output Single-Inductor DC-DC Buck Converter with Self-Boosted Switch Drivers and 1.2 A Total Output Current”, Solid-State Circuits Conference, 2008, ISSCC 2008, Digest of Technical Papers, IEEE International, Issue Date: 3-7 Feb. 2008, pp. 444-626 (ISBN: 978-1-4244-2010-0; INSPEC Accession Number: 10047949; Digital Object Identifier: 10.1109/ISSCC.2008.4523248), teaches the use of the anti-crossing phase of a SIMO (Single Inductor Multiple Output) DC-DC Buck converter to create a boost voltage used by the output low side (LS MOSFET). This solution, however, requires expensive internal and external capacitors.